1. Technical Field
The invention relates to a method for the early detection of aerodynamic instabilities in a turbomachine compressor. It applies to any type of turbomachine and particularly to aircraft jet engines.
Under certain operating conditions, the compressor of a turbomachine may go into an unstable mode which causes damage ranging from a simple reduction in the working life of the parts of the engine to a complete stalling of the engine which may give rise to an accident. Aerodynamic instabilities, known by the name of revolving stall and surging, are conventionally encountered when the variations in engine operating conditions lead to an abrupt change in one of the compressor parameters such as the mass flow rate of fluid, the temperature or the outlet pressure.
Surging is characterized by mainly axial fluctuations in the flow through the compressor.
Revolving stall is a phenomenon which produces one or more localized zones, known as stall cells, which propagate in the circumferential direction in the cascades of the compressor blades at a fraction of the rotational speed of the compressor.
Depending on the nature of the cells, a distinction is drawn between part span stall and full span stall. Part span stall may be made up of several rotating cells constituting local perturbations which simultaneously affect only a restricted number of blades and which radially are restricted to a fraction of the compressor flow path.
In contrast, full span stall affects a radially and angularly larger region of the cascades of blades with greater variations in amplitude between healthy zones and stalled zones.
In general, the rate of propagation of the stall cells is higher in the case of a part span stall than in the case of a full span stall. The rate of propagation of the stall cells is of the order of 70 to 80% of the rotational speed of the compressor rotor in the case of a part span stall and about 40 to 50% in the case of a full span stall.
A distinction is drawn between two types of aerodynamic instability precursors known respectively as local perturbations and modal perturbations, and it is necessary to have effective methods of detection available for these two types of precursor.
Local perturbations are generally observed a few tens or a few hundreds of compressor revolutions before the instability extends to the entire compressor. However, they are difficult to detect because they are restricted to a limited number of blades and to a fraction of the span of the compressor flow path. They are of small amplitude and of highly random behavior: they may occur almost instantly, propagate for a fraction of a compressor revolution and then disappear without giving rise to any irreversible overall compressor instability. Furthermore, these local perturbations are characteristic of a situation for which, on approaching a zone of instability, a particular stage of the compressor reaches its aerodynamic load limit before the other stages, which entails the sensors preferably being placed near to this stage in order to be able to detect these local perturbations.
Modal perturbations also occur a few tens or a few hundreds of compressor revolutions before the instability. They are directly associated with the dynamic response to the fluctuations of the fluid flow in the entire compressor when the compressor reaches the maximum value on its characteristic curve expressing pressure as a function of flow rate. These modal perturbations are thus characteristic of a situation for which the various stages of the compressor together reach their aerodynamic load limit. Modal perturbations affect the entire circumference of the compressor at a given moment and are therefore relatively easier to detect than local perturbations because they are found at all the sensors.
2. Summary of the Prior Art
To solve the problems of aerodynamic instability, it is known practice to install preventive mechanical systems that make it possible to avoid the onset of instability. These mechanical systems may, for example, be adjustable inlet guide vanes, adjustable stators, and blow-off valves placed inside the compressors. These preventive systems do, however, impose operating constraints and significant limits on performance.
There also exist curative systems which make it possible to detect a surge situation and get out of it.
The technique of actively controlling instabilities which is used in most recent civilian and military engines consists of detecting an instability cycle when the latter occurs, and using this detection to trigger corrective action on compressor or engine parameters so as to get out of this zone of instability under the best possible conditions.
This technique does, however, carry a risk of damaging the engine and of leading to the loss of the aircraft on which it is mounted.
To improve the performance of engines while at the same time ensuring satisfactory levels of safety, it is known practice to employ another technique for actively controlling instabilities which allows the compressor operating point to remain for as long as possible in a zone close to the zone of unstable operation.
This technique for actively controlling instabilities consists of detecting precursors of aerodynamic instabilities which occur before the compressor enters a cycle of instability. These precursors may, for example, be variations in pressure, speed, or temperature which are detected in signals from sensors several compressor revolutions before the compressor enters a cycle of instability. If detection can be made early enough, corrective action to correct the operating point or the geometry of the compressor can be carried out before the instability actually occurs. This technique of actively controlling instabilities has the advantage that it is possible to operate with a far smaller surge margin because in real time it allows these instabilities to be prevented. However, the current methods of detecting instability precursors are difficult to implement and do not perform well because they are generally associated with the development of a detection threshold applied to the amplitude of the signals measured in a very noisy environment.
WO96/34207 describes a method that makes it possible to detect aerodynamic instability precursors and which consists of measuring the energy of the frequency signal at the frequency of rotation of the compressor, and then comparing this energy to an empirical threshold value developed from observations of signals obtained in healthy engines during acceleration.
However, only the energy of the signal at the frequency of rotation of the compressor is analyzed, and this is unable to detect all the precursors of instabilities which may occur at frequencies other than the rotational frequency of the compressor.
This is particularly troublesome because the rate of propagation of local perturbations is often lower than the rotational speed of the compressor.
The object of the invention is to determine a novel method for the early detection of aerodynamic instabilities in a turbomachine compressor which performs well, effectively, is simple to implement, and enables any perturbation to be detected regardless of its speed of propagation.
Accordingly, the invention provides a method for the early detection of instabilities in an aerodynamic flow in a turbomachine compressor including a rotor carrying rows of moving blades which rotate between rows of fixed blades, said method comprising the steps of:
a) collecting signals over a predetermined length of time from at least one sensor disposed at an angular position e on the circumference of the compressor between successive rows of fixed and moving blades;
b) normalizing the signals collected;
c) detecting, from the normalized signals, events for which the normalized signals have a property consisting of an absence of passage through a zero value during a length of time Tz at least greater than the time Ta taken for two successive moving blades to pass said sensor;
d) analyzing all of the detected events; and
e) making a decision regarding the imminence of instability in the aerodynamic flow through the compressor according to the result of the analysis of the detected events.
In particular, the invention provides a method for detecting local perturbations which occur very early on in signals provided by pressure sensors or flow rate sensors arranged more or less uniformly around the circumference of the compressor between the rows of fixed and moving blades. The method involves detecting events characteristic of instability precursors and applying decision criteria to decide on the imminence of aerodynamic instability in the compressor. The decision may be taken on criteria based on the rate of detection of events characteristic of instability precursors. Alternatively, the decision may be taken on criteria based on the spatio-temporal position of the events detected across all of the sensors and on the evaluation of a level of probability of the presence of propagation of the local perturbations detected. The decision may also be taken on the above two criteria applied in succession or in parallel.
More particularly, in a first embodiment of the invention, the analysis of the detected events consists of:
counting the events detected by the sensors during a predetermined length of time greater than the length of time Ta taken for two successive blades to travel past the sensor so as to obtain an event detection rate; and
comparing the detection rate value obtained with a predetermined threshold value so as to obtain at least a first indication about the imminence of instability.
In a second embodiment of the invention, the analysis of the detected events consists of:
determining the spatio-temporal location of the events detected from the normalized signals collected from at least two sensors distributed uniformly around the circumference of the compressor;
searching, in a time/azimuth frame (t,xcex8), for the directions of propagation of the events corresponding to propagation rates of between a zero value and a value equal to the rotational speed of the compressor rotor;
allocating, to each direction of propagation found, and to each event detected, a level of probability relating to the presence of spatial propagation of the detected event in the found direction; and
comparing each level of probability with a second predetermined threshold value so as to obtain at least one indication about the imminence of instability.
In a third embodiment of the invention, the analysis of the detected events consists of:
counting the events detected by the sensors during a predetermined length of time greater than the length of time Ta taken for two successive blades to travel past the sensor so as to obtain an event detection rate;
comparing the detection rate value obtained with a first predetermined threshold value so as to obtain at least a first indication about the imminence of instability;
determining the spatio-temporal location of the events detected from the normalized signals collected from at least two sensors distributed uniformly around the circumference of the compressor;
searching, in a time/azimuth frame (t,xcex8), for the directions of propagation of the events corresponding to propagation rates of between a zero value and a value equal to the rotational speed of the compressor rotor;
allocating, to each direction of propagation found, and to each event detected, a level of probability relating to the presence of spatial propagation of the detected event in the found direction; and
comparing each level of probability with a second predetermined threshold value so as to obtain at least one indication about the imminence of instability.
Other preferred features and advantages of the invention will become apparent from the following description of the preferred embodiments, which is given by way of non limiting example and with reference to the appended drawings.